1. Field of the Invention
The present invention relates to a fixing device that is used on an image forming apparatus for obtaining a fixed image by fixing a developer image formed on a fixing medium.
2. Description of the Related Art
There are so far available such type of fixing devices comprising an image forming apparatus as those devices equipped with a heating roller and a pressure roller. The heating roller heats an image fixing medium, that is a paper, carrying a developer image formed thereon by a powder developer. The pressure roller is in contact with this heating roller via a fixing medium and conveys the fixing medium while applying the pressure on it.
When a fixing medium passes through a contacting portion (called as the nip) between these heating roller and pressure roller, a developer on this fixing medium is fused, press fitted and fixed.
So far, such a fixing device uses a halogen lamp, etc. as a heating source and a heating roller is composed by providing this halogen lamp, etc. in a metallic roller. In addition to this type of heating source, there are some fixing devices provided with a flush lamp and heat a fixing medium by lighting this lamp without contacting the fixing medium.
However, a lamp used as a heating source once converts electric energy into light and heats and applies them to a metallic roller by the radiant action and therefore, efficiency is worse and thermal efficiency is limited to about 70%.
Further, as much time is needed to warm-up the fixing at present, it is largely demanded to make the warm-up time short. So, there is a view to increase the nicer of lamps as a heat source from one to two pieces but this will not only make an apparatus large in size but also increase the power consumption.
In view of above, in order to make the start-up time short, several technologies to fix an image using the induction heating had been developed. For instance, the Japanese Patent Gazette No. 8-76620 disclosed an apparatus to fix a developer image on a recording medium by closely fitting this recording medium to a conductive film that is heated by a magnetic field generating means. Further, a technology to heat a thin metallic layer of a roller provided around a cylindrical ceramics by applying induction current using an induction coil has been disclosed in the Japanese Patent Gazette No. 59-33476.
However, these technologies have such a defect that a start-up time can be made short but are applicable only to copying machines that are operated at a low speed because there is available no heat storage element. Further, in order to run a film, a complicated control including a zigzag control is required and a cost will increase.
In the case of such an inducting heating type fixing device, it is desirable to use a strong magnetic material for heating a film by generating eddy current efficiently and an iron material is generally used.
An iron material has lower thermal conductivity than aluminum and copper materials, heat history tends to be left and the heat build-up characteristic of a heating means tends directly to appear. Accordingly, if a thin iron made roller is used, uneven temperatures are produced depending on accuracy of the position or heat build-up characteristic of said heating means.
Further, the shape of magnetic flux produced differs slightly at the central part and the end of a heating roller. So, a temperature becomes different between the central part and the end of the heating roller and the escape of heat from the end is larger than the central part and thus, a temperature difference is produced on a roller.
Further, such a problem was also caused that if an image fixing medium in smaller width than the fixing nip width is passed, a temperature difference is produced between the passed portion and the non-passed portion, this temperature difference is left as a temperature history and the uneven fixing will result.
In addition, on a fixing device utilizing the electromagnetic induction heating as described above, there was such problem shown below.
Core elements wound round heating coils are formed in one united body by cutting or using a mold and it was difficult to get a dimensional accuracy in the longitudinal direction. In particular, when a ferrite element is used for the core element, it was difficult to form a core in a complicated shape in one united body and if dimensional accuracy is demanded, a manufacturing cost will increase sharply. Further, if conductive materials (iron core, permalloy, amorphous metal) are used for core element when forming core elements in one united body, eddy current is generated on the core elements themselves and heat is generated by the core element itself. In other words, thermal efficiency drops.
Further, a core element formed in one united body has such problems that it is not possible to regulate a heat value in the longitudinal direction and the volume of heat radiated to the air at the end portion becomes larger than that at the central part of the nip portion, and the heating at the end portion becomes insufficient. Further, when an image fixing medium in the width smaller than that of the fixing nip was passed through the nip portion, a temperature difference is produced between the passed portion and the non-passed portion of the paper become different and while leaving this temperature difference as a temperature history, an uneven fixing was also-produced.
In case of a means to solve this problem as disclosed in the Japanese Patent Gazette No. 8-16005, a plurality of core elements are arranged in one direction and this state of arrangement is held by a holder. According to this structure, it is possible to prevent generation of eddy current in the core elements themselves. However, the structure with core elements simply arranged in one direction is often performed for cores of transformers, etc. and the basic structure to prevent generation of eddy current in core elements. Further, the cores arranged and held by a holder only may cause insufficient strength and it becomes difficult to maintain a distance to an image fixing medium accurately. Further, as rotary components are in the positions close to the cores, the structural strength is needed and the insufficient strength results from the structure to hold the arranged cores by a holder. In addition, there was a problem that individual core elements are oscillated in the holder by the oscillation accompanied with the rotation. Further, there were also such problems that a magnetic field generating means including core elements becomes a large size as a result of the holder provided around the core elements and it becomes difficult to downsize the unit, etc.